Throughout the world, and particularly in Europe, a metering signal is used to continuously notify a customer of telephone charges. The metering signal is usually a 16 kilohertz or 12 kilohertz signal sent from a central office to a subscriber line.
The central office (office exchange) generally includes a line card having a subscriber line interface circuit (SLIC device) and a subscriber line audio-processing circuit (such as SLAC.TM. or DSLAC.TM. devices manufactured by Advanced Micro Devices). The SLIC device is an analog interface for the subscriber line generally including audio amplifiers for providing audio signals to a subscriber line. The subscriber line is generally coupled to a subscriber telephone or other customer communications equipment. The SLAC device is generally coupled to the SLIC device to transmit and receive analog signals and provides digital signals to the central office. The SLAC device generally includes analog-to-digital (A/D) converters and digital-to-analog (D/A) converters.
Generally, the line cards or other equipment in the central office are designed to provide a metering signal to the subscriber line. The voiceband impedance of the linecard is generally required by the telephone administration to be 600 ohms or greater. Unless a complex circuit is coupled with the SLIC device, the impedance of the line card is about 400 ohms or more at the metering frequency (12 or 16 KHz). The impedance of the subscriber line varies from 200 ohms to 3,000 ohms at the metering frequency. The variance in impedance is generally due to the variation in the impedance of subscriber meters and to the variable subscriber line lengths from the central office to the customer telephone or other equipment.
The voltages of the metering signal at the subscriber line vary by about three to one due to the impedance variations unless a special circuit is included to control the variability. These large variations in voltages can disadvantageously overload the audio amplifiers in the SLIC device. Also, these large variations in voltages may exceed the maximum voltage allowed by the administration.
High impedance of the subscriber line at the metering frequency (often up to 3,000 ohms) also may create large echoes or reflections of the metering signal. The voltage of echoes and reflections of the metering signal is zero at a critical value of the line impedance which is referred to as the metering balance impedance. This value is preferably close to 200 .angle.-30.degree. ohms at the metering frequency.
A high impedance meter connected through a short subscriber line has an impedance far larger than the metering balance impedance. Such a high impedance of the subscriber line may cause the SLIC device or SLAC device to saturate or interrupt normal transmission and reception on the subscriber line. Also, the reflections of the metering signal at the subscriber line may also exceed the maximum voltage allowed by the administration of the telephone line.
Heretofore, communications systems have employed a transformer-based metering signal injection circuit to provide the metering signal and a filter in the transmit path to filter out the large reflections of the metering signal. The filter generally filters out all signals having a 12 kilohertz or a 16 kilohertz frequency. The signal injection transformer and filter are analog components placed on the subscriber line side or customer side of the transmission path to avoid saturating the amplifiers and converters in the SLIC device or SLAC device.
Metering signal injection transformers and filters coupled to the subscriber line (line interface) are generally costly because they require additional analog circuitry. Also, they include high voltage components which must withstand voltages up to 2,000 volts due to lightning or power crosses. Metering filters often require at least second order filtering so that only the metering signal is filtered out without disturbing the voiceband and so that the metering source has a low impedance at the metering frequency and a high impedance in the audio band. Second order (or higher) filtering is costly because precision components are needed.
Further, filters such as notch filters generally cannot be designed for both 12 kilohertz and 16 kilohertz metering signals. Additional cost is involved in a notch filter solution because no one notch filter design can accommodate both commonly used metering signal frequencies. Thus, the economies which may be realized in manufacturing a single part to address both metering signals are not available.
It would, therefore, be advantageous to generate the metering signal within the SLAC device with a programmable metering signal generator. It would also be advantageous to have a single metering signal level control apparatus which could employ low cost parts to mitigate noise caused by metering signals at all commonly used frequencies. It would also be advantageous to utilize the internal amplifiers in the SLIC device to provide the metering signal to the subscriber line.